This invention relates to a measuring and operating circuit for a Coriolis mass flowmeter.
Coriolis mass flowmeters are widely used to determine the mass flow rate of a fluid in a section of pipe. The fluid passes through at least one vibrating flow tube. In most Coriolis mass flowmeters, a vibration exciter and two vibration sensors are mounted on the flow tube. The flow tube and the fluid together form a vibratory system which is normally excited at its resonance frequency. The resonance frequency depends, among other things, on the material and dimensions of the flow tube. It also varies with the density of the flowing fluid. In some cases, the flow tube is excited not at the resonance frequency, but at an adjacent frequency.
The two vibration sensors sense the vibratory motion of the flow tube at two locations spaced a given distance apart in the direction of fluid flow, and convert this vibratory motion into sensor signals. Both sensor signals have the same frequency as the vibratory motion of the flow tube, but they are out of phase.
The phase difference is a measure of the mass flow rate. In a measuring subcircuit, the sensor signals are evaluated and converted to a signal proportional to the mass flow rate of the fluid. Aside from the mass flow rate, further properties of the fluid, e.g. its density, can be determined. This is accomplished, for example, by evaluating the frequency of the vibratory motion of the flow tube.
U.S. Pat. No. 4,801,897 describes an excitation subcircuit which is constructed in the manner of an analog phase-locked loop. In that circuit, the excitation frequency adjusts itself automatically to the resonance frequency of the vibratory system even during variations in fluid density.
The prior-art measuring circuits use either analog techniques, as described in EP-A 698 783 or U.S. Pat. No. 4,895,030, for example, or digital techniques, as described in EP-A 702 212 or U.S. Pat. No. 5,429,002, for example.
EP-A 698 783 discloses a measuring circuit comprising an analog control loop which regulates the two sensor signals at the same amplitude.
EP-A 866 319 discloses a further measuring and operating circuit. In this circuit, the two sensor signals are amplified before being processed, with one of the amplifiers having a variable gain.
In a digital processor, the sum and difference of the two sensor signals as well as one of the sensors signals are evaluated.
For the accuracy of the measurement it is essential that after their amplification, the two sensor signals have the same amplitude. The amplitude regulator required for this purpose evaluates the sum and difference of the two sensor signals.
For the actual determination of the mass flow rate, in addition to the difference signal, the signal from one of the two sensors is needed.
Altogether, in this circuit, three analog vibration signals are formed and then processed in an arithmetic unit. For each vibration signal, at least one A/D converter is necessary in the arithmetic unit.
It is an object of the invention to provide an operating circuit for a Coriolis mass flowmeter in which fewer vibration signals have to be formed and evaluated and which nevertheless has sufficient accuracy and is easy and inexpensive to implement.
This object is attained by a measuring and operating circuit for a Coriolis mass flowmeter comprising a transducer assembly with at least one flow tube on which a first and a second vibration sensor, spaced a given distance apart in the direction of fluid flow, and a vibration exciter are mounted, the measuring and operating circuit comprising: a first amplifier, which is connected to the first vibration sensor; a second amplifier, which is connected to the second vibration sensor; a first A/D converter for generating a vibration signal S1 proportional to the output signal of the first vibration sensor, which is connected to the first amplifier; a difference stage having its two inputs connected to the first amplifier and the second amplifier, respectively; a second A/D converter, following the difference stage, for generating a difference signal D proportional to the difference of the amplified output signals from the first and second vibration sensors; and a digital processor which, of the vibration sensor signals, evaluates only the difference signal D and the sensor signal S1, and which performs the following steps:
a) Determining the amplitude AMS1 of the sensor signal S1
b) Determining the in-phase component I and the quadrature components Q of the difference signal D with respect to the sensor signal S1 as a reference signal
c) Controlling the gain of the second amplifier in such a way that the in-phase component I of the difference signal disappears
d) Calculating the mass flow rate via the remaining quadrature component Q according to the formula
mxcx9cQ/(AMS1*f).